In Xenopus laevis, vitellogenin is encoded in a small family of related genes and is synthesized in the liver under control of estrogen. Genomic vitellogenin sequences were isolated from a permanent library of the Xenopus laevis genome. They were characterized by analysis with restriction endonucleases and heteroduplex mapping in the electron microscope. The gene library was constructed by joining large random fragments of DNA to the lambda vector charon 4 with help of synthetic DNA linkers. Very high numbers of different recombinant clones could be obtained by using the in vitro packaging procedures.